1. Field of the Invention
The present invention relates to a spin tunnel magnetoresistive effect element for reading a magnetic field strength, which is information recorded on a magnetic medium or the like, as a signal, and to a spin tunnel magnetoresistive effect film used therein, and more particularly it relates to a spin tunnel magnetoresistive effect film, a spin tunnel magnetoresistive effect element, and a spin tunnel magnetoresistive effect sensor and magnetic apparatus using the element, that achieve a large output with a small external magnetic field.
2. Related Art
A ferromagnetic tunnel junction has a construction in which a tunnel barrier layer made of an insulation film having a thickness of several nanometers is sandwiched between two ferromagnetic layers. In this element, if a certain current caused to flow between the ferromagnetic layers and an external magnetic field is applied within the ferromagnetic plane, there appears magnetoresistive effect, in which the value of resistance changes in response to the relative angle of magnetization between the magnetic layers. If the magnetizations are parallel, the resistance value is minimum, and if the magnetization is anti-parallel, the resistance value varies in accordance with the angle thereof, and when the magnetization orientations are opposite, the resistance is maximum.
Therefore, if a coercivity difference is imparted to the magnetic layers, it is possible to establish parallelness or anti-parallelness of magnetizations responsive to the strength of an external magnetic field, making it possible to detect a magnetic field by the change of the resistance value. If the polarization of the two magnetic layers are P1 and P2, the resistance value change ratio, which establishes the magnetic field sensitivity, is expressed as 2 P1 P2/(1−P1 P2). This equation means that the larger the polarizations are of both magnetic layers, the larger is the magnetoresistive change ratio. In recent years, with an improvement in the quality of tunnel barrier layers, it has become possible to achieve a ferromagnetic tunnel junction exhibiting a magnetoresistive change ratio close of 20%, which is close to the theoretical value, resulting in an increase in the possibility for application to magnetic heads and magnetic memories.
Typical reported examples of such large magnetoresistive change ratio are found in Journal of Applied Physics, Vol. 79, pp. 4724–4729 (1996) and Journal of Applied Physics, Vol. 81, pp. 3741–3746. In these reported examples, a 20-nm Pt electrode is laminated onto a silicon substrate, over which are formed a 4-nm NiFe layer, a 10-nm FeMn layer, a 8-nm NiFe layer, a 1-to-3-nm Al layer, after which oxygen plasma is used to form an Al oxide film, followed by subjecting the 8-nm NiFe layer to an exchange coupling magnetic field. In this element, a high magnetoresistive change ratio of 22% is achieved.
In Applied Physic Letters, Vol. 72, pp. 605–607 (1998), there is a report of a ferromagnetic tunnel junction element made of Co/Al2O3/Co/NiO using NiO as an anti-ferromagnetic material and having magnetoresistive change ratio of 17% in a room temperature. In IEEE Transactions of Magnetics, Vol. 33, pp. 3553–3555 (1997), there is a report of a ferromagnetic tunnel junction element made of NiFe/Co/Al—AlOx/Co/NiFe/FeMn/NiFe using FeMn as an anti-ferromagnetic material, in which a magnetoresistive change ratio of 24% is observed at room temperature, the magnetic hysteresis thereof being similar to that of a spin valve.
A method of fixing magnetization using an anti-ferromagnetic material such as noted above has been used in the past in a spin valve film, and these reported examples could be said to be applications of this method to a ferromagnetic tunnel junction.
The magnetic hysteresis in the element is similar to that of a spin valve film, and there is a reduction in hysteresis in the zero-field region. Compared to a coercivity difference type ferromagnetic tunnel junction, therefore, this configuration is suitable for application to devices such as magnetic sensors.
In order to apply a ferromagnetic tunnel junction element to a high-density magnetic recording head, it is desirable in this manner that one of the ferromagnetic materials have a spin-valve structure with exchange bias applied thereto. The anti-ferromagnetic material used therein not only must have thermal stability that does not hinder device operation, but also must have a high resistance to corrosion in the device fabrication process. In the above-noted reports, however, when low-blocking-temperature of FeMn (blocking temperature 150° C.) and NiO (blocking temperature 200° C.) are used, the thermal stability is insufficient.
The reason for this is that, if the blocking temperature is low, the temperature rise occurring when the element operates weakens the pinned magnetic field, so that as a result of environmental magnetic fields (such as a magnetic field by a sensor current or the like), there is a change in the pinned layer magnetization direction, resulting in a drop in magnetic field sensitivity therein. In the case of an anti-ferromagnetic material having a high blocking temperature, even if the temperature of the element rises, it is difficult for the magnetization direction of the pinned layer to change, which would cause a decrease in sensitivity. FeMn in particular has poor corrosion resistance, and there are remaining problems to be solved in the device fabrication process as well.
Mn regular alloys of PtMn, PdMn, and NiMn are anti-ferromagnetic materials having a high blocking temperature of 300° C. or greater. An anti-ferromagnetic material made of these Mn regular alloys has superior thermal stability due to its high blocking temperature, and also has good corrosion resistance, making this material extremely advantageous when a ferromagnetic tunnel junction element is applied to a device such as a magnetic head.
These materials, however, do not exhibit an exchange coupling magnetic field in the condition immediately after film formation. The reason for this is that, in the condition immediately after film formation, these materials are in a chaotic phase.
Therefore, in order to regularize the chaotic phase so that a proper exchange coupling magnetic field is achieved, it is necessary to perform thermal processing in a magnetic field for a long period of time at a higher temperature than in the past (250° C. for PtMn, 230° C. for PdMn, and 270° C. or higher for NiMn, for a period of approximately 5 hours).
In a spin tunnel magnetoresistive effect element in the above-noted literature, although operation is done with a small external magnetic field, in the case of using these in practical sensors and magnetic heads, there is the problem that the neel temperature of FeMn is low, leading to a problem of poor thermal stability in the device. If a substance such as PtMn, PdMn, or NiMn or the like, having a high neel temperature is used as an anti-ferromagnetic film, it is necessary to perform proper thermal processing in order to achieve an anti-ferromagnetic phase (regular phase), and this thermal processing causes problems such as diffusion of oxygen or nitrogen within the tunnel barrier layer, and a reduction of the resistance change ratio, that is, a reduction in the output when used as a device.
In a method of fabricating a spin tunnel magnetoresistive effect film of the past, the resistance of the spin tunnel element was 100 Ω, which is extremely high, and the influence of deterioration of the high-frequency response in the magnetoresistive detection system and the influence of shot noise impedes the achievement of a sufficient S/N ratio at high recording densities.
If the tunnel barrier layer is made thin, so as to reduce the resistance of the element in order to handle the above-noted problem, there is the problem that current leakage occurs because of the pinhole effect, thereby lowering the magnetoresistive change ratio.
While the disclosure of Japanese unexamined patent publication (KOKAI) No. 2000-215415 is known, although this publication has language with regard to a magnetoresistive effect element in which the surface roughness of the lower shield layer is made 3 nm or smaller, in this technology even if the surface roughness of the lower shield layer is made smaller than 3 nm, there is little substantial influence on the surface of the tunnel barrier, and it is not possible to achieve the intended magnetoresistive effect element.
Accordingly, it is an object of the present invention to provide a spin tunnel magnetoresistive effect film and spin tunnel magnetoresistive effect element magnetoresistive change ratio of which is not lowered even when the thickness of the tunnel barrier layer made thin and having superior high-frequency response and thermal stability, sufficiently low resistance for application to a magnetic head, and a high linear magnetic field sensitivity in the region about the zero field point.